Wednesday, June 27, 2018

The biggest, brightest lasers make for good headlines, but this isn’t a story about those. This is a story about lasers so tiny you need a microscope just to see them—lasers smaller than red blood cells. These tiny lasers could play an important role in next-generation medical care (among other technologies), and that makes them a big deal.

Created by a team of researchers from the United States, Italy, and Kazakhstan working at Lawrence Berkeley National Laboratory (LBNL), the “microlasers” can shine for several hours, even when submerged in blood serum or water. They are small enough, say the researchers in a journal article published this week in Nature Nanotechnology, “to be embedded in organisms, tissues or devices.”—potentially even in living brain tissue.

When scientists create something like a laser or a refrigerator on a small scale, it’s worth considering exactly what the term means—they aren’t usually talking about a miniaturized version of the iconic technology. The word laser describes devices on many scales, from the small lasers in DVD players to stadium-sized research lasers and even astrophysical processes. There are lasers that produce red light, as well as lasers that produce green, blue, x-ray, infrared, and microwave light. There are lasers that emit light continuously and lasers that emit light in pulses.

So what makes a laser a laser?

Lasers produce intense, focused beams of coherent light. This is because unlike flashlights, lasers emit light waves that are nearly identical to one another. On a fundamental level, this requires (1) a bunch of atoms, (2) an outside source of energy that excites the atoms and causes them to produce light, and (3) a container for the atoms and light. Once you get one atom to emit light, nearby atoms are inspired to do the same, and the effect multiples quickly. To get a laser beam, you just create an opening in the container so that some of the light can escape. Check out the minutephysics video How lasers work (in theory) for a nice, quick illustration of the basics.

Each type of laser has its own design challenges. When it comes to making tiny lasers, one of the main challenges is efficiency. When you decrease the container size, a greater fraction of the light leaks out. To make up for this, you need to put more energy into the system, but this makes the device more prone to damage. Under the direction of Bruce Cohen, Emory Chan, and James Schuck from LBNL, the new research aimed to bypass these problems with three innovations.

As the light-producing elements, the researchers started with nanoparticles made from sodium, yttrium, and fluoride. Then, they doped the nanoparticles with a key ingredient—thulium. Previous research suggested that with the addition of thulium, this kind of nanoparticle could absorb infrared light, get excited, and produce light with a higher energy. Put simply, these nanoparticles are more efficient at producing laser light than you’d expect when they're illuminated with infrared light, and they produce a kind of laser light that is great for technological and biological applications.

As the container, the researchers used a tiny polymer bead. Rather than escaping or being absorbed when it hits the inside edge of one of these beads, the scientists found that most of the light produced by nanoparticles inside of the bead stayed inside—reflecting along the inner surface (a form of total internal reflection). These light waves interacted to produce an especially bright signal around a specific wavelength. This, in turn, stimulated nanoparticles to produce even more light in that range.

The third innovation brought it all together. The researchers embedded the nanoparticles inside of the beads in a particular way that made it especially easy for the light to get trapped. Each microlaser consisted of one bead with a halo of nanoparticles just under its surface.

An image (left) of a 5-micron-diameter bead coated with nanoparticles, and an image (right) that shows a cross-section of a bead, with nanoparticles along its outer surface. The scale bar at left is 1 micron, and the scale bar at right is 20 nanometers. Image Credit: Angel Fernandez-Bravo/Berkeley Lab.

Next, the researchers imaged and tested the lasers to see what they could do. The results show that they worked well—producing the expected type of laser light and doing so efficiently. The lasers could shine continuously for more than five hours, even when immersed in blood serum. They are easy to make, store, and use, say the authors, and demonstrate the feasibility of creating new microlaser-based tools that could someday make their way into a variety of places, even into your body*.

Many people think of lasers as destructive tools, and they can be, but this work has less violent potential. For example, the light produced by these microlasers depends on the shape and size of the cavity—the bead. These features are sensitive to a microlaser’s environment, so a by monitoring the light emitted by a microlaser embedded in human tissue, you could theoretically detect slight changes in tissue temperature or pressure. The microlasers also have the potentially for imaging, tracking, probing, and even trapping things like proteins and DNA.

“Reducing the size of lasers to microscale dimensions enables new technologies that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors to live brain tissue,” explain the authors in the article. Lasers in the brain—that’s something to wrap your mind around.

*Although this work represents an important advance toward technologies like optogenetics, it's important to remember that there's still a long way to go; to get these beads to lase, the study's authors had to illuminate them with infrared light at an intensity of more than 10 kW/cm2, which is enough to burn a hole in you pretty quickly. This is down from previous thresholds of 1010W/cm2, so if we continue to see similar improvements in coming years, the technology may yet find its way into the clinic before too long.